Non-volatile memory devices are an important element of integrated circuits due to their ability to maintain data absent a power supply. Phase change materials have been investigated for use in non-volatile memory cells. Phase change memory cells include phase change materials, such as chalcogenide compounds, which are capable of stably transitioning between physical states (e.g., amorphous, semi-amorphous, and crystalline states). Each physical state exhibits a particular resistance that may be used to distinguish logic values of the memory cell.
A conventional phase change memory cell includes a layer of phase change material between a first electrode and a second electrode. A portion of the phase change memory cell is set to a particular resistance state according to an amount of current applied by the first electrode and the second electrode. To obtain an amorphous state, a high current pulse is applied to the phase change material for a short period of time to melt a portion of the phase change material. The current is removed and the phase change memory cell cools rapidly to below the glass transition temperature of the phase change material. To obtain a crystalline state, a lower current pulse is applied to the phase change material for a longer period of time to heat the phase change material to below its melting point, causing the amorphous state to re-crystallize to a crystalline state that remains after the current is removed.
Conventional phase change memory cells typically also include an isolation element. Within a conventional phase change memory device, a plurality of phase change memory cells are positioned between a plurality of access lines (e.g., word lines) and a plurality of digit lines (e.g., bit lines). Each access line is connected to a row of phase change memory cells and each digit line is connected to a column of phase change memory cells. A single phase change memory cell is selected for reading or writing by applying a voltage between the access line and the digit line to which the single phase change memory cell is connected. Including an isolation element in the phase change memory cell impairs or, ideally, prevents residual voltages from affecting the physical state of non-selected phase change memory cells. Threshold switching materials are currently considered favorable isolation elements. One threshold switching material is a chalcogenide compound having an OFF state that is relatively resistive (i.e., precluding current leaks) and an ON state that is relatively conductive. The ON state of the threshold switching material is enabled when a voltage across the threshold switching material is greater than a critical threshold value of the threshold switching material.
Disadvantageously, conventional phase change memory devices require large amounts of energy to generate the heat to produce a detectable resistance change in the plurality of phase change memory cells. The electrodes used to generate such heat are often insufficiently resistive. Conventional electrodes also frequently exhibit poor resistivity stability over varied temperatures and have a tendency to be chemically reactive with the phase change material of the phase change memory cell, which can result in limited memory cell lifespan, performance degradation, and even delamination of the phase change material. Additionally, conventional electrode materials do not exhibit good adhesion with phase change materials. Further, conventional electrodes often have textured, uneven, or rough surfaces that can protrude through a portion of the phase change material and negatively affect memory characteristics.
There remains a need for a phase change memory cell and a phase change memory device providing the benefits of a threshold switching material without one or more of the foregoing disadvantages of conventional phase change memory cells and devices.